The Survival Pack for Left 4 Dead is set to be released for Xbox 360 on April 21, 2009.
Shacknews explains what the Survival Pack will bring to L4D:
The free Survival Pack adds the new Survival game mode, where players are given time to dig in before fending off unceasing zombie hordes in an attempt to survive as long as possible. Survival Mode is played on 12 new maps--11 being sections of existing levels and 1 created especially for Survival Mode, Lighthouse.
The Survival Pack also brings Versus mode to the final two co-op campaigns, Dead Air and Death Toll. This release coincides with the release of Left 4 Dead: Critics Choice Edition, which contains the original game plus the Survival Pack.
The Survival Pack will be free for all Left 4 Dead players.